As bandwidth demand for indoor wireless users continues to grow, cellular operators are trying to explore bandwidth provisioned from indoor, in addition to providing bandwidth from outdoor. Because of the physical nature of radio signals, however, cellular operators have faced difficulties to provide full coverage for indoor users. Cellular repeater is one of the common solutions for current systems, but it may degrade received signal quality and has no intelligence for signal processing. Relay station is another solution developed to resolve this problem, but there is no commercial relay station available yet and it is still under development. Femtocell is yet another solution that is developed to enhance indoor coverage by reusing the licensed spectrum as part of the cellular network infrastructure.
FIG. 1 (Prior Art) illustrates a simplified cellular network 10 that comprises a macro base station BS11 and a femto base station BS12. Cellular network 10 also comprises an outdoor mobile station MS14 and an indoor mobile station MS15. As illustrated in FIG. 1, macro BS11 provides strong signal strength to outdoor MS14, while provides relatively weak signal strength to indoor MS15 because of physical obstruction and/or reflection caused by building 13. On the other hand, femto BS12, an access-point base station (e.g., a small indoor base station), is able to provide strong signal strength and good signal quality to indoor MS15 because femto BS12 is located inside building 13.
Femtocell is anticipated to be an important feature to support extreme high-speed transmission for 4G systems. Both IEEE 802.16m and 3GPP RAN1&RAN2 are currently developing femtocell technology as part of the standards for WiMAX 2.0 and LTE-Advanced systems. Extreme high-speed transmission will result in very high power consumption and is usually used to support multimedia services, which are more possible be requested by users at indoor environment. By using femto base stations, more radio resources can be saved by using shorter range and lower transmission power. FIG. 2 (Prior Art) illustrates system architecture of a WiMAX femtocell system 20.
Network synchronization of downlink transmission timing in a cellular network is usually performed by Global Positioning System (GPS). GPS is a global navigation satellite system that provides reliable positioning, navigation, and timing service. However, a femto BS may not be able to receive GPS signals and obtain timing reference. FIG. 3 (Prior Art) illustrates a cellular network 30 that comprises a GPS31. Cellular network 30 also comprises macro BS32 and BS33, as well as a femto BS34. As illustrated in FIG. 3, BS32 and BS33 are able to receive GPS signals from GPS31, while BS34 is not able to receive GPS signals and obtain timing reference because it is located inside building 35.
In addition to GPS, backhaul signaling may also help to achieve network synchronization among difference BSs. However, backhaul connection of a femto BS is not reliable for obtaining timing reference. FIG. 4 (Prior Art) illustrates a backhaul connection of a femto BS in a WiMAX femtocell system 40. As illustrated in FIG. 4, Femto BS backhaul is expected to be low-cost xDSL or DOCSIS link. It is not as robust and reliable as dedicated connections used in Macro-/Micro-/Pico-BS. In addition, the round trip delay may be time variant and result in difficulty on precise timing refinement. Thus, downlink network synchronization for femtocell in a cellular orthogonal frequency division multiplexing (OFDM) and/or orthogonal frequency division multiple access (OFDMA) systems remains a challenge.